1. Field of the Invention
The present invention relates to array antennas for communications systems, particularly RF microcell personal communications systems.
2. State of the Art
Wireless personal communications systems are known as exemplified by published International Application WO 96-41498 entitled Hearing Aid With Wireless Remote Processor, incorporated herein by reference. As described therein, a hearing aid system consists of an earpiece that can be hidden in the ear canal, and which communicates wirelessly with a remote processor unit (RPU). The RPU may be a belt pack, wallet or purse-based unit. Sounds from the environment are picked up by a microphone in the earpiece and sent with other information over a primary two-way wireless link to the RPU, where the audio signals are enhanced according to the user's needs. Signal processing is performed in the RPU rather than the earpiece to take advantage of relaxed size and power constraints. The enhanced audio signals may be combined with other information and transmitted from the RPU over the primary wireless link to the earpiece, where they are converted by a speaker to sounds that can only be heard by the user.
In an exemplary embodiment, communications between the RPU and the earpiece follow an interrogate/reply cycle. The reply portion of the primary wireless link (from the earpiece to the RPU) may use a reflective backscatter technique in which the RPU radiates a carrier signal and the earpiece uses a switch to change between a high backscatter antennas state and a low backscatter antenna state. An additional, optional secondary two-way wireless link can be used for communication between the RPU and a cellular telephone system or other source of information. Furthermore, an RPU keyboard, or voice recognition capabilities in the RPU, can be used to control hearing aid parameters and telephone dialing functions. Two earpieces and an RPU can be used in a binaural wireless system that provides hearing protection and noise cancellation simultaneous with hearing aid functions.
Although the system of WO 96-41498 arises out of the field of hearing health care, as may be appreciated from the foregoing description, the system is more broadly applicable to personal communications in general. Recently, attention has been drawn to the application of wireless personal communications systems to telecommunications and computing. At "ACM97: The Next 50 Years of Computing", for example, the prediction was made that in the future, personal computers will be wrist-sized, accompanied by a pair of reading glasses that present high-resolution images at a comfortable distance. A small, fitted earpiece and a "finger mouse" will be linked to other devices with low-power radio signals. Such a future is not far off.
One of the challenges presented in personal communications systems is to allow multiple such systems to function in close proximity to one another with no performance degradation (or graceful degradation) due to interference. An unofficial benchmark developed by the present assignee to test for robustness of communications in the presence of interference has been the "ten-person hug. " That is, ten persons each with a personal communications system of the type described should be able to form a group hug without experiencing significant performance degradation of their respective personal communications systems.
In a personal communications system as described, the RPU requires an antenna diversity system to mitigate against signal drop out due to signal nulls encountered in any real-world situation. Basically, the signal emanating from the earpiece antenna may reach the RPU's receiving antennas via numerous paths, due to multiple reflections from environmental objects. These reflections result in "multipath" problems.
Classical antenna diversity systems employ more than one antennas and either a) when the signal quality is measured to be below a predetermined threshold, the receiver input is switched to a different receiving antenna (with, hopefully, a better quality signal) or b) each antenna has its own receiver and the best quality received signal is utilized as the output signal. Any of various different measures of signal quality may be employed, such as signal strength, bit-error rate (BER), signal distortion, etc. Typically the antennas are spaced physically apart so that if one is in a null, the other or others are unlikely to also be in a null. A conventional diversity antenna system in accordance with the former technique is shown in FIG. 1. A conventional diversity antenna system in accordance with the latter technique is shown in FIG. 2.
In the first case a) active switching circuitry must be located in the antenna's signal path where signals are small and weak and subject to degradation by the switch. Furthermore, data transmission or reception must be interrupted periodically to perform a comparison of the signals received by the different antennas. Based on this comparison, one of the signals is selected. Such comparison, or "hunting," uses bandwidth that might otherwise be used for data transmission or reception. In the second case b) multiple receivers are required with the increase in size, weight, power, complexity, and, of course, cost.
In diversity antenna systems, multiple antennas function independently, usually without significant RF interaction. Apart from diversity antenna systems, directional antenna systems are also known. In directional antenna systems, also known as "beam steering" or "beam forming" antenna systems, the RF interaction between multiple antennas is controlled to realize the equivalent of a single antenna having a desired directionality. Directional antenna systems are most commonly used in radar applications, but are also being increasingly used in cellular communications, for example.
In some instances, passive reflector elements have been used to generate directionality. Referring to FIG. 3, for example, a linear antenna 31 forming a driven element has positioned adjacent to it a thin reflector element 33. With respect to the driven element, the reflector dipole is shorted out to cause the reflection of energy and is mistuned to a lower frequency (by using a longer element) to provide a phase delay that compensates for the reflective-to-active-element spacing d, thereby causing maximum radiation in the desired direction. Such a configuration is not adaptive and cannot be used to improve reception in a rapidly-changing RF environment.
A limited measure of adaptivity is attained using a conventional phased array antennas system of a type shown in FIG. 4. Multiple antennae 41 are coupled together using transmission lines (1.sub.1 -, 1.sub.2, 1.sub.3). The transmission lines function as delay lines, the lengths of the transmission lines being chosen to exhibit the desired delay. Two different sets of transmission lines are provided, the transmission lines in each set having length chosen appropriately to achieve a desired directionality. RF switches 43 are used to switch between the two different sets of transmissions lines. When the RF switches are in one state, for example, the antennas system might be optimized for "broadside" reception. When the RF switches are in the other state, the system might be optimized for 45.degree. reception. The limited degree of adaptivity of the system of FIG. 4 comes at the expense of increased size and cost.
Other conventional phased array antennas systems are fully adaptive. Referring to FIG. 5, for example, multiple antenna elements 51 are each coupled to individual phase shifters 53 and antenuators 55, the outputs of which are coupled to a common line feed 57. Referring to FIG. 6, a conventional phased array antenna system is shown using continuously adjustable RF phase shifters 61 and separate receivers (63, 65) for each element. (The separate receivers are provided with a common frequency reference f.sub.0, element 64.) Using RF signal processing techniques, the signals from the two different elements (67, 69) can be summed (block 68) in any desired phase relationship.
None of the foregoing techniques is suitable for a compact, low-power, low-cost personal communications system. What is needed, then, is an antenna system that provides the benefits of known diversity and/or directional antenna systems but that is small, power efficient, and low-cost. The present invention addresses this need.